1. Field of the Invention
The present invention relates generally to load responsive hydraulic systems in which the effective output of the pump is controlled by the difference between the load actuating pressure of a fluid motor and the pump pressure; that is, the effective output of the pump is controlled to provide the required pump flow at a pressure that is at a predetermined pressure magnitude above the load actuating pressure. In systems using fixed displacement pumps, the effective output of the pump is controlled by bypassing excess pump flow to a sump; whereas, in variable displacement or variable discharge pumps, a displacement control mechanism directly controls the output of the pump.
More particularly, the present invention relates to load responsive systems that utilize a synthetic signal generator to pressurize this signal fluid to a predetermined pressure magnitude above the load actuating pressure of the fluid motor. This synthetic signal principle is fully disclosed in U.S. Pat. No. 3,971,216, of common assignee.
2. Description of the Prior Art
In U.S. Pat. No. 3,971,216, of common assignee, a load responsive hydraulic system is disclosed in which a signal fluid is supplied by the pump, is pressurized by flowing through a synthetic signal generator which comprises a relief valve or an orifice, and flows on into a fluid motor in opposition to the load actuating pressure therein. The result is that this signal fluid is pressurized to a synthetic signal pressure which is at a predetermined pressure magnitude above the load actuating pressure.
In load responsive hydraulic systems of the synthetic signal generator type, as described above, and as disclosed in the reference patent, when the directional control valve is actuated from a standby position to an operating position, the directional control valve must provide one fluid flow path for the synthetic signal fluid to flow to one motor port of the fluid motor and another fluid flow path from the other motor port to a sump before blocking the flow of the signal fluid to a sump. Otherwise, the flow of signal fluid will be momentarily blocked as the directional control valve is moved from the standby position to an operating position, the signal fluid will be excessively pressurized by this blocking, and the pump will be excessively pressurized momentarily. The present invention provides pressure actuated means for alleviating this timing problem in the directional control valve.